I think this is the best parable - a master key with no notches.

I'd refine this to, the cylinder for the lock did not care what key you put in, it woul open it anyways. So the key looks secure, but the lock is a joke.

You can use a lollipop to open it.

Except it's still not good enough, because you could use that key to eavesdrop nearly anything and anyone.